The invention relates to a method for determining the state of charge (SOC) of a battery whether in an equilibrium (rest) or active state by measuring on a non-invasive basis the battery open circuit voltage (OCV) and other parameters.
It is important to know what percentage of a battery""s full energy capacity, measured in amp-hours (Ah), is available at any given time. The capacity percentage is generally called the state of charge (SOC). The SOC directly reflects the capability of a battery and hence the system or application that it powers. Knowing battery SOC is especially important for mission critical applications such as an uninterruptible power source (UPS) application. In such an application, it must be determined when the battery SOC has dropped below an acceptable level so that it can be recharged or replaced.
Several conventional methods exist for determining the state of charge (SOC) of a battery. For a lead-acid battery, one of these methods is the use of electrochemical means of determination. This generally is a method for determining SOC by using the measurements of electrical properties of the battery, such as its open circuit voltage (OCV), which is the voltage measured across the battery terminals with no load on the battery. This is done without making any invasive, physical measurements, e.g., specific gravity of the electrolyte. In addition, this method does not base its value on historic battery performance (i.e. coulomb counting), but on an instantaneous analytic technique.
One method for determining battery SOC based on measuring SOC is described in U.S. Pat. No. 4,754,349, which is owned by the assignee of the subject application. However, in an electrochemical battery, such as of the lead-acid type, polarization of the plates of the battery as well as instantaneous battery condition can affect the functional relationship between SOC and OCV. This may then affect the ability to make an accurate SOC determination by measuring only its OCV. Also, the straightforward determination of SOC based on measured OCV is inaccurate to the degree that the battery is not in a xe2x80x9crestedxe2x80x9d state. A xe2x80x9crestedxe2x80x9d state is achieved when the battery has had an opportunity to achieve chemical equilibrium after having undergone a full or partial charge or discharge. When a battery reaches a state of chemical equilibrium, the battery also comes into electrical equilibrium. In some cases, it takes up to several hours for a battery to reach its fully rested state after its charging or discharging is terminated.
A need exists for a method of rapid and accurate electrochemical type determination of SOC of a battery, such as one of the lead acid type, to be extended to points at which the battery is not in an ideal, rested state. This will permit rapid determination of battery SOC under a wider range of its operating conditions, while maintaining measurement accuracy.
The present invention relates to a method for determining battery state of charge (SOC) at any time, without having to wait for the battery to settle to a rested state. The invention accomplishes high accuracy SOC measurement, without requiring substantial waiting time before the SOC determination can be made. The invention provides a substantially instantaneous information capability to the SOC determination, and hence provides a mission critical battery system (e.g., UPS system) with a high degree of readiness.
In accordance with the invention, a first type algorithm is developed that relates the battery SOC to the OCV of a battery in its rested state. To do this, a battery is tested by charging and discharging it over a cycle from 0% to 100% and back to 0% and stopping at different values of SOC, e.g., 10%, 20%, 30% . . . 100% during both the charge and discharge portions of the cycle. At each value of SOC the charge or discharge is stopped and the battery is permitted to rest for a time, for example, 2-3 hours, to reach its settled condition. This time is hereafter referred to as the settling period. During the settling period at each SOC value, the battery OCV, rate of change of OCV and battery case temperature are measured until the battery reaches its fully settled state.
The OCV is measured at the end of the settling period for each SOC value. The voltage measured at this time is hereafter referred to as OCVREST. A first type algorithm is developed from a plot of the data of the OCVREST versus the various SOC values.
From the monitored OCV, rate of change of OCV and battery case temperature data acquired during the settling period at each of the SOC values, at least one second type algorithm is developed of a predicted rest state OCV, hereafter referred to as OCVPRED. The OCVPRED is then used in the first type OCVREST vs. SOC algorithm to determine the battery SOC as if the battery was in the rested state. In a preferred embodiment of the invention, two second type OCVPRED value algorithms are developed for different ranges of battery SOC.
In accordance with the invention, to determine the SOC of a battery under test, it is only necessary to measure its OCV. If the battery is in the settled, or rest state, then the OCV can be used directly with the first type algorithm to determine its SOC. If the battery being tested is still in the active (not settled) state, then the measured actual SOC, rate of change of SOC and battery case temperature is used in a second type algorithm to determine an OCVPRED voltage that is then used with the first type algorithm to determine the battery SOC as if it is in the settled state.
It is therefore an object of the invention to provide a method to determine the available energy capacity percentage (SOC) of a battery by measuring its OCV.
Another object is to provide a method to determine battery SOC by measuring its OCV without having to wait for the battery to reach a rested state.
Yet a further object is to provide a method to determine battery SOC on an instantaneous basis by measuring its OCV and without having to use invasive measurements.
An additional object is to determine the SOC of a battery by measuring its OCV and using the measured OCV value with algorithms that relate SOC to OCV for any condition of the battery.